Transformer



March 21, 19 39. R Q E Y 2,151,035

TRANSFORMER I Filed Dec. 11, 1937 ATTORNEY.

Patented Mar. 21, 1939 UNITED STATES TRANSFORMER Theodore B. Kennedy.

Morrisville, Pa., assignor to Ajax Electrothermic Corporation, AjaxPark. N. 3., a corporation of New Jersey Application December 11. 1937,Serial No. 179,219

'lClaims.

This invention deals with transformers and specifically thosetransformers which are used or useful in rapid inductive heatingapparatus.

An object of the invention is to provide a transformer in which theratio of turns between pri-' mary and secondary is high, yet in whichthe impedance of the secondary is low.

A further object is to provide a focus inductor transformer wherein theimpedance of the power collecting portion is matched against theimpedance of the distributing inductor.

A further object is to provide a transformer with a secondary windingcomprising fractional parts of a single turn.

A further object is to provide a method for increasing the current flowin a focus inductor distributing block without materially increasing theprimary voltage.

Further purposes will'appear in or be evident from the specification.

Attention is directed to applicants copending application, Serial No.252,906, filed January 26, 1939, which is a continuation in part of thisapplication and in which are claimed certain features of applicant'sinvention referred to, but not claimed herein.

Applicant has chosen six figures by way of illustration.

Figures 1 and 2 are diagrammatic views of focus inductor circuitsshowing the use of inside and outside primary coils with one turnsecondary coils.

Figure 3 is an isometric view of an inductive heating device embodyingapplicant's invention. Diagrammatically shown in connection with thisfigure is an alternating current power source and power factorcorrection therefor.

Figures 4, 5 and 6 are diagrammatic views of transformer circuitsaccording to this invention, showing respectively half turn, third turnand quarter turn secondary windings. In Figure 5 is shown how an ironcore might be placed in a transformer of the type described.

The focus inductor as described in Northrup U. S. Patent 1,378,187, andelsewhere, is well known. It comprises a transformer and distributinginductor for concentrating a large block of power into a comparativelysmall charge piece. The primary of the transformer, like most heatinginductors, usually comprises a single layer helical coil of hollowcopper tubing arranged for water cooling. The turns usually are edgewound to get a large number of turns into a small space. The secondaryusually comprises a single electrical turn closely coupled with theprimary and lying either inside or'just outside of same. If thesecondary is of broad width it usually is subdivided withcircumferentially directed openings to cut down eddy current losses. Thedistributing inductor, like the secondary, is usually of one turn, butmay be of several turns. Ordinarily it comprises a single loop, usuallya hollow casting of rugged construction and small size, adapted in formto lie closely within or around the charge to be heated. It is connectedin series with the secondary and usually is adapted to carry anextremely heavy high frequency alternating current. Because of the highfrequency of the current used in a focus inductor assembly an iron coreusually is not used. It may be used either in the main transformer or inthe distributing inductor, or in both if desired.

The value of a focus inductor heating assembly lies in its ability toperform intricate, fast or unusual heating, although its efficiency isrelatively low with respect to other methods of heating. Assuming thatthe best efiiciency in ordinary induction heating is from 50 to percent, the focus inductor eiiiciency is of the order of 15 to 30 percent, and since many applications to which the focus inductor may beapplied are not of the type offering good electrical coupling,efficiencies have been known to be as low as 5 per cent. Whileefliciency is of general, it is not of essential importance; and even incases where the efficiency is as low as 5 per cent the use of a focusinductor is sometimes considered commercially sound. This is truebecause many of the applications to which the focus inductor is appliedcan be handled in no other way. The present invention describes a methodof improving the efficiency of certain types of focus inductorequipment.

Applicant has found that as the coupling between the distributinginductor and the charge piece becomes poor, due to the small size of thecharge piece or to its irregularity orphysical make-up, the efllciencyof the system falls off. This he attributed to the fact that with afocus inductor assembly of the usual construction the distributinginductor, instead of transmitting power to the load, forms substantiallya short circuit on the secondary winding of the transformer; andbecause'fiif the poor coupling between the primary and secondarywindings, causes the voltage of the secondary to fall off. Applicantdevised his present construction to gain a better voltage ratio underload conditions between primary and secondary.

By dividing the one turn secondary into a plurality of symmetricalfractional turn windings, each in parallel with the other, and in serieswith the distributing inductor, he was able to decrease the resistance,and hence the impedance of the secondary, without sacrificing the totalcurrent flow in the distributing inductor. In fact, because of the lowersecondary impedance, the current as a whole was increased, with anaccompanying better regulation and higher overall efficiency.

Assuming that each segment of the fractional turn secondary has inducedin it l/nth of the current which would be induced in a full turnsecondary (n being the total number of segments), then all n turnsoperating in parallel would supply the same current as with the one turnsecondary. Forgetting that the reentrant portions of each segment haveresistance, the resistance of the total 11 segments, operating inparallel is reduced to l/n th of the resistance of the single turn. Thusby applicant's method, even considering some resistance in the reentrantportions of the secondary segments, the total impedance is greatlyreduced over what it would have been using a complete turn.

Applicant has found that the number of parts into which he can divide afocus inductor secondary with constructive results is dependent upon theultimate load conditions. As a general rule, if the diameter of thecharge piece is small the number of segments in the secondary to obtainthe best results will be large, and vice versa. Applicant believes thatthe impedance of the distributing inductor, including the load, shouldbe equal to the transformer secondary reactance for the best results,and he has effectively checked his conclusions with a number ofexperiments involving commercial size equipment. In his tests secondarycoils having from one to four segments were experimented with and thesewere compared with the more common type focus inductor assemblies forefliciency and practical operation data. Improvements in efliciency upto 30 per cent were recorded.

In Figure 1 applicant has shown a focus inductor assembly of the usualconstruction. In this figure an alternating current source I, feeds theprimary coil of a focus inductor transformer 2. Capacitors 3 are used toresonate the coil. The secondary of the transformer 4, is shown in thiscase as surrounding the primary. The distributing inductor is shown bythe loop 5.

The assembly shown in Figure 2 is similar to that of Figure 1 with theexception that the primary 2 surrounds the secondary 6. Figures 1 and 2are used by way of illustration to show for comparison some of thefeatures of the older type focus inductor assemblies. The matter ofinternal and external coils is considered to be old, and can be appliedequally well to applicant's present invention.

Applicant's invention is best illustrated by Figure 3 in which a focusinductor assembly is shown having a transformer secondary, each sectionof which comprises a third part of a turn. In this figure a highfrequency power source i is connected, as in Figure 1, to the primary ofthe transformer l. The primary comprises a single layer helical coilclosely coupled to the secondary 8. Insulation, not shown, would beplaced between primary and secondary windings.

The secondary 0, comprises three parts, 9, l0 and II, which togethermake one complete electrical turn, and each part is connected inparallel with the other two. For instance, re-entrant parts l2, l3 andI4 are connected to the center connecting block i5, while parts l8, l1and I! are connected through connecting studs I9, 20 (not shown) and 2|to connecting blocks 22 and 23. The parts of the secondary are dividedcircumferentially by spaces 24. Insulation, not shown, would be placedbetween re-entrant portions I2-I6, 13-41 and ll-IS, and also between thecentral connecting piece l5 and the other connecting studs 19, 20 and2|.

The distributing inductor block 25 is adapted with bolts 26 for use inclamping same to the connecting blocks 15 and 23 of the transformersecondary.

In Figures 4, 5 and 6 applicant has shown primary windings 2 surroundingtwo, three and four part secondaries 21, 28 and 29 respectively. In eachof these figures one end of each secondary section is connected to acommon Junction, and it is assumed that the free ends would be similarlyconnected together as shown in Figure 3. In Figure 5 applicant has shownhow an iron core 30 might be applied to a transformer of the typedescribed.

Theoretically, there is no limit to the number of segments which can beused in a transformer of this type if the load condition justifies thedegree of subdivision; and for this reason applicant does not wish to belimited to the half, third and quarter turn examples used forillustration.

Applicant has described his invention with particular reference toinductive heating apparatus, but believes it to be equally applicable tothe design of transformers for other uses.

What is claimed follows:

1. A transformer comprising a winding with a plurality of electricalturns, and a winding with a plurality of conductors connected inparallel at their extremities each coupled only fractionally with thecircumferential part of said first mentioned winding but as a groupsubstantially fully coupled with same.

2. A transformer comprising a winding with a plurality of electricalturns and a winding comprising one physical turn divided into aplurality of are like sections, said sections being electricallyconnected in parallel and each being coupled with a differentcircumferential portion of said first winding.

3. A transformer winding comprising substantially a single layer hollowcylinder divided circumferentially to limit eddy currents andlongitudinally to effect a plurality of fractional turns, the fractionalturns being electrically connected in current additive parallelrelation.

4. In a focus inductor assembly a transformer primary, a transformersecondary comprising a plurality of fractional turns each inductivelycoupled with a different circumferential portion of the primary circuitand connected in parallel to a distributing inductor.

5. A transformer comprising a primary winding having turns ofsubstantially 360 degrees of arc and a secondary winding comprising aplurality of conductors, connected in parallel at their extremities,each electrically insulated from the others and inductively coupled withthe primary only over from 20 to 50 per cent of its circumferentiallength, these conductors as a group being inductively coupled with theprimary over substantially in its full circumferential length,

6. A transformer comprising one winding having a substantially annularturn and a second winding comprising a plurality of conductors,

connected in parallel at their extremities, each inductively coupledwith said primary turn only over a portion of its annular length but asa group inductively coupled with said turn over substantially its fulllength.

7 A transformer having a helical type primary winding, a secondarycomprising a plurality of conductors each inductively coupled with thesame longitudinal but a different circumterential portion of the primarycircuit and connected in parallel to a distributing inductor, the numberof conductors in said secondary, together with their physicalcharacteristics, being so chosen as to make their combined impedancesubstantially equal to the impedance of the distributing inductor.

THEODORE R. KENNEDY.

